Doping features

Tungsten-rhenium thermocouples are available to measure the highest temperatures. They appeared in the USSR since 1956. An admixture of 5% rhenium to the positive TP electrode increases its plasticity and recrystallization temperature. In order to obtain the maximum possible thermo-EMF the proportion of rhenium in the negative thermoelectrode should be as different as possible from the content in the positive one. But at 32% of rhenium and more in W-Re alloy above 1100 °C intermetallic phases are formed and accuracy of sensor readings drops. Such an alloy will be inhomogeneous - when heated significantly, rhenium will evaporate, which also increases the error. Therefore, in the USA they use 25-26% rhenium alloy, and in the USSR - 20% alloy. Nevertheless, the accuracy of the BP5/20 thermocouple at t° 900-2100°C decreases linearly. In order to increase the stability of BP thermocouples are alloyed with silicon-alkali additives (0,1-0,5% KCl, 0,1-0,5% Al;, O3, 0,1-0,5% SiO2). This decreases the error for high temperature measurements by a factor of almost 3.

Features of graduation

The calibration table for the thermocouple VR5/20 was developed in 1968. The increase in the error due to non-uniformity of VP electrodes and insulation shunting at high temperatures was investigated most thoroughly by Prof. Dr. Gordov (VNIIM). The specifications of GP thermocouples production were adopted in 1973 and included in GOST 6616-74. In 1977 GOST 3044-74 was also included in the standardized NSKh (nominal static characteristic), developed on the basis of the temperature scale MPTSh-68.

Graduation of the thermocouple VR5/20 was carried out in several organizations: in Yekaterinburg at the Ural Institute of Metrology, in Lvov at KB "Termopribor" and in Podolsk at NPO "Luch". We determined the MTF of thermocouples in a vacuum furnace by melting wires of silver, copper, nickel, platinum, rhodium, iridium, tantalum, wound on the working junction of a thermocouple. Up to the melting point of platinum, the RMS error of graduation was estimated at the point of rhodium. Readings of GP-thermocouple were compared with thermocouple PR30/6 (up to 1800°С) and optical pyrometer. It was found that the graduation of 80% of HP-thermocouples had a scatter of values up to 1%, the minimum error for the thermocouple BP5/20 of class 2 was set at the level of error of the thermocouple type B (PR30/6) 0.5%. For a class 3 BP thermocouple, the error is greater than 0.5%.

Fluctuations of percentage composition

According to specifications for thermocouple wire, it is allowed to vary the percentage of rhenium in the alloy up to 0.5% and the percentage of impurities up to 0.1%. As a consequence, there is a scatter of thermal EMF in different batches of wire. One coil of wire may have a scatter of thermal EMF up to 50 μV at 1500 °C.
GOST 3044-77 standardizes three close calibrations for the thermocouple BP5/20: A-1, A-2 and A-3. Graduations A-2 and A-3 are respectively located above and below the main graduation A-1 with a working range of 1000 to 2500 ° C and go only to 1800 ° C.

Production

In the 1980s, approximately 150 kg of BP5 (BAP5) and BP20 thermocouple wires were produced per year. The wire was used for the needs of scientific-research institutes and aircraft-building design offices, was used in the technology of composite materials production. The main consumer was metallurgy. When measuring the temperature of the molten metal, BP thermocouples successfully replaced expensive platinum-rhodium thermocouples both in the Soviet Union and in CMEA countries. The collapse of the socialist camp in the early 90's led to a crisis in industrial production and a 60% drop in thermocouple wire consumption. Now the recovery of production requires more and more reliable high-temperature gauges, such as BP thermocouples.

Today about 90 kg of 0.35-0.5 mm BP5 and BP20 thermocouple wire with a diameter and a total length of up to 20 km are produced annually. The alloy is produced by powder metallurgy. A mixture of tungsten powder and ammonium salt perrenate (NH4ReO4) is pressed into stacks, then alloyed and rolled into wire. Because of the "floating" content of rhenium in the BP5 and BP20 alloys, it has not yet been possible to drastically optimize the thermoelectric characteristics of the thermocouple. However, recent advances in micrometallurgy, the advent of alloying nano-additives and improvements in metal analysis have already begun to improve the situation.

Use

These thermocouples are only used in a vacuum or inert environment. To extend their application, a special sealed sheath filled with inert gas was developed to protect thermocouples in a carbon environment (e.g. vacuum furnace with graphite heaters) and in oxidizing environments. For example, a sealed thermocouple in a molybdenum sheath is designed for temperature control in a vacuum furnace for sintering fuel pellets, its operating life is up to 1000 hours at 1750 °C in an aggressive environment. Other sealed VR-temperature transducers in aluminum (leucosapphire) monocrystal shrouds with 2000-3000 h operating life at 1600 °C in oxidizing media have been developed. They are also used in thermal probes to monitor the temperature of molten metals, salts or glass.

Such transducers are used in highly corrosive environments and the service life depends only on the durability of the protective sheath. At temperatures of 1000 - 1700 °C they replace platinum-rhodium thermocouples if the permissible error is not less than 0.5%.

Thermocouple Wire Qualification

Conducted in a conventional high-temperature furnace. Thermocouples from pieces of BP wire taken from the beginning and end of the coils are placed in a casing of leucosapphire and sealed there. Calibration is carried out in the range of 600-1700°C by reference thermocouple PR30/6, placed in the same furnace. This method, which is simpler and cheaper than vacuum furnace calibration, is used for up to 90% of thermocouple wire calibration applications. Because VR-thermocouples are quite reliable, and often the only possible, means of measuring temperature in the range of 1500-2500°C they are included in the CIS standards and are in demand everywhere at the present time.

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